Add missing index to records table

Updates a relevant test to remove flakyness

priv/repo/migrations/20241022205853_add_purchased_at_index_to_records.exs

@@ -0,0 +1,7 @@
+defmodule MusicLibrary.Repo.Migrations.AddPurchasedAtIndexToRecords do
+  use Ecto.Migration
+
+  def change do
+    create index("records", [:purchased_at])
+  end
+end